After the terrorist attacks of Sep. 11, 2001, there has been an increasing focus in the United States, as well as the rest of the world, to protect against future terrorist attacks. Unlike several years ago, protection from terrorist attacks is not just limited to conventional weapons such as small arms, light weapons and cluster munitions. Now terrorists are becoming quite sophisticated and radiological and/or nuclear attacks have become serious threats.
To successfully fight against radiological and nuclear terrorism, it is critical that radiological or nuclear materials that are either in transit or in a device which has been planted are detected. As background radiation in the environment is common and there are many innocent materials that are radioactive this process can be difficult. Further complicating this process is the need for the flow of commerce to be unimpeded and not unnecessarily alarm the public.
One type of conventional detector for detecting radiological or nuclear materials utilizes large cylindrical gas proportional counters that are filled with Helium-3 (He-3) gas, surrounded by thermalization material, to detect fission neutrons. The thermalization material acts to convert incident fast neutrons to thermal neutrons which are efficiently detected by He-3. Descriptions of He-3 gas proportional counters (and other similar gas counters) are given in standard reference books (e.g. G. F. Knoll, Radiation Detection and Measurement, third edition (John Wiley & Sons, New Jersey, 1999) Chapter 14.)
He-3 is a non-radioactive isotope of helium that is extremely sensitive at detecting neutron radiation. When thermal neutrons interact with the gas, charged particles are formed, which can be easily ascertained by the sensors. However, due to the recent world shortage of He-3 gas, there is a need to replace He-3 gas by another method of detecting thermal neutrons efficiently.
One approach is to deploy a thin layer of Boron-10 (B-10), which has a strong affinity for thermal neutrons and converts the thermal neutrons into charged particles that result from the nuclear interaction. The charged particles are then detected by the gas filling a conventional proportional counter. U.S. Pat. No. 7,952,078 B2 (May 31, 2011) describes an optimization of the B-10 coating to achieve increased detection efficiency.
In view of the He-3 shortage, an entirely different mechanism that provides detection sensitivity and efficiency comparable to a conventional He-3 proportional countered is needed. A gas avalanche neutron detector (GAND) that employs a thin B-10 layer as a neutron converter is one approach to provide an extremely sensitive detector neutron radiation without the use of He-3 gas.